The field of the invention is nuclear magnetic resonance imaging (MRI) methods and systems. More particularly, the invention relates to MR imaging of the brain.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
There are many MR imaging techniques used to acquire diagnostic information from the brain. These include contrast enhanced T1-weighted images that brightly reveal regions where the blood-brain barrier is destroyed, T2-weighted fast-spin-echo (FSE) and fluid attenuated inversion-recovery (FLAIR) imaging which show the extent of edema surrounding a damaged region. Two of the most important diagnostic tools, however, are diffusion-weighted imaging (DWI) and perfusion-weighted imaging (PWI) which measure physiological parameters that correlate with tissue health.
Diffusion-weighted imaging (DWI) is a powerful MRI technique for probing microscopic tissue structure. In DWI, a pulse sequence is employed which contains a magnetic field gradient known as a diffusion gradient that sensitizes the MR signal to spin motion. In a DWI pulse sequence the detected MR signal intensity decreases with the speed of water diffusion in a given volume of tissue. The first moment of this diffusion gradient, also known as the “b-value” determines the speed of diffusion to which the image is sensitive. This b-value may be adjusted by either varying the area of the two lobes of the diffusion magnetic field gradient, or by varying the time interval between them. When water motion in the subject is unrestricted, the MR signal intensity at the center of the echo using a spin-echo diffusion-weighted pulse sequence is related to the b-value as follows:
                    A        =                                            S              ⁡                              (                b                )                                                    S              0                                =                      ⅇ                          -              bD                                                          (        1        )            where the “b-value” b=γ2G2δ2(Δ−δ/3). The parameter γ is the gyromagnetic ratio and G is the amplitude of the applied diffusion magnetic field gradients. S(b) is the MR signal magnitude with diffusion weighting b, and S0 is the MR signal magnitude with no diffusion weighting (b=0). The parameter D is the diffusion coefficient of the fluid (in mm2/s), which directly reflects the fluid viscosity where there are no structural restrictions to diffusion of the water. Δ is the time interval between the onsets of the two diffusion gradient lobes and δ is the duration of each gradient lobe. The diffusion coefficient D in equation (1) may be calculated, since b is known and the attenuation A can be measured.
Nerve tissue in human beings and other mammals includes neurons with elongated axonal portions arranged to form neural fibers or fiber bundles along which electrochemical signals are transmitted. In the brain, for example, functional areas defined by very high neural densities are typically linked by structurally complex neural networks of axonal fiber bundles. The axonal fiber bundles and other fibrous material are substantially surrounded by other tissue.
Diagnosis of neural diseases, planning for brain surgery, and other neurologically related clinical activities as well as research activities on brain functioning can benefit from non-invasive imaging and tracking of the axonal fibers and fiber bundles. In particular, diffusion tensor magnetic resonance imaging (DTI) such as that disclosed in U.S. Pat. Nos. 6,526,305; 6,642,7126 and 6,806,705 has been shown to provide image contrast that correlates with axonal fiber bundles. In the DTI technique, motion sensitizing magnetic field gradients are applied in a diffusion weighted imaging (DWI) pulse sequence so that the magnetic resonance images include contrast related to the diffusion of water or other fluid molecules. By applying the diffusion gradients in selected directions during the excitation/imaging sequence, diffusion weighted images are acquired from which apparent diffusion tensor coefficients are obtained for each voxel location in image space. From this tensor the dominant direction of spin diffusion in each voxel can be determined.
Fluid molecules diffuse more readily along the direction of the axonal fiber bundle as compared with directions partially or totally orthogonal to the fibers. Hence, the directionality and anisotropy of the apparent diffusion coefficients tend to correlate with the direction of the axonal fibers and fiber bundles. Using iterative tracking methods, axonal fibers or fiber bundles can be tracked or segmented using the DTI data as described in U.S. Pat. No. 6,526,305.
To calculate the apparent diffusion tensor coefficients, it is necessary to acquire at least six DWI images using motion-sensitizing gradients directed in six different directions. Using the DTI method of measuring anisotropy in each image voxel will indicate a single direction that corresponds to a single neuronal fiber. This method is very limited in its ability to track neuronal fibers that cross each other in an image voxel.
Diffusion sensitive imagery (DSI) is a method for analyzing DWI images to determine the diffusion directions in each image voxel. As described in U.S. Pat. Nos. 6,614,226 and 7,034,531, by acquiring more DWI images at different directions and gradient b values, it is possible to produce an image that indicates multiple diffusion directions in each image voxel. When a fiber tracking method is used with such a DSI image, neuronal fibers that cross each other can more accurately be detected and displayed.